1. Field of the Invention
The present invention relates to an apparatus for tonal image processing, and more particularly to such an apparatus, for example a laser beam printer providing an image with half-tone, with improved resolution and gradation.
2. Description of the Prior Art
Scanning systems utilizing a rotary polygonal mirror or vibrating mirror are commonly employed in various displays facsimile, printing apparatus etc. utilizing a laser source because of the advantages of large scanning angle and small color dispersion, and particularly those utilizing a rotary polygonal mirror are used as high-speed scanning apparatus.
In order to record or display an image with continuous or half-tone in such a scanning system, there are known the systematic dither method and the density method which are effective for forming an image with half-tone with a recording or displaying medium only capable of representing white and black.
FIG. 1 schematically shows such systematic dither method. Quantized and sampled input image signal x generally has density information of K bits per pixel and is compared for each pixel with a threshold value C to provide a binary output signal 1 or 0 respectively when it is larger or smaller than said threshold value C, wherein said threshold value assumes one of the values of K bits. In this manner the input image signal of K bits/pixel is converted by the comparator into binary image signal of 1 bit/pixel.
The threshold values C are generally represented in a matrix corresponding to the pixels. In the systematic dither method proposed is Judice's matrix which assumes the following form for each pixel in case the pixel is composed of 2.times.2=4 dots or micro-pixels: ##EQU1## or the following form in case the pixel is composed of 4.times.4=16 dots: ##EQU2## wherein the area of each dot is equal to 1/4 of each pixel in case of the matrix (1) or 1/16 in case of the matrix (2). Generally speaking the gradation becomes richer as the dimension of the matrix is increased, and an n.times.n matrix can represent n.sup.2 +1 steps including zero level.
The half-tone representation by such method has the advantages of:
(1) simplicity in system as each dot can be recorded in two steps of black or white; and PA1 (2) the resulting possibility of using a photosensitive member with non-linear characteristics: PA1 (1) poor resolution as each pixel is composed of plural dots; and PA1 (2) a larger pixel area required for obtaining improved gradation, leading to further deteriorated resolution.
while it is associated with the disadvantages of: